When you have a certain domain added in the allowlist/denylist but disabled, and you try to add it again through the allow/deny popup, it says that the domain already exists, then you have to go to the respective list, look for the domain and finally toggle it.
This feature would cut all this process and make the user able to toggle this domain right from the allow/deny popup.
An alternative method would be silently toggling the domain when it already exists and notify the user about that.